The machining speed of a wire electric discharge machine is greatly affected by wire disconnection. Namely, precise machining cannot be effected under conditions in which the frequency of wire cut is high, and accordingly, a machining operation has to be carried out under conditions (low cutting speed, low discharge current)less severe than such machining conditions.
Generally, wire disconnection is considered to be caused because the wire temperature locally rises and thus the wire strength lowers there, as a result of concentrated electric discharge during the machining operation, and it has been confirmed that after concentrated electric discharge actually occurs at a portion of a wire, the wire is disconnected at that portion. Therefore, efforts have been made to avoid concentrated electric discharge, for example, to control the on-off time of a machining power supply.
The most important factor in the phenomenon of wire disconnection is considered to be the wire temperature and the temperature distribution during a machining operation. Specifically, if the local temperature rise of the wire is moderate during machining operation, then it is necessary that the entire wire be cooled more intensely. Conversely, if the local temperature rise of the wire is extremely high, it is necessary to distribute the electric discharge. In any event, it is at present difficult to directly measure the wire temperature, and effective measures still remain unfound.
A first method currently implemented for measuring the wire temperature is a method in which the wire temperature is estimated based on the temperature-strength characteristic of the wire. When the wire is disconnected under certain conditions, the temperature of the disconnected portion of the wire is estimated from the tension then acting on the wire.
A second method is a method based on analysis, wherein the wire temperature is calculated based on an assumed random heating of the wire along the length thereof and an assumed heat transfer coefficient of a wire surface.
According to the first method, however, only the temperature of the disconnected portion of the wire is estimated, and it is not possible to find the temperature of the entire wire or the temperature distribution.
The second method does not take account of the effect produced by the flow of cooling water in the machining groove, which provides a dominant wire cooling effect. Thus, estimation of the heat transfer coefficient is not as effective as expected, and actual wire temperature cannot be calculated with high accuracy.